Evidence for multiple effects of ProTxII on activation gating in Na V1.5

Abstract

The peptide toxin ProTxII, recently isolated from the venom of the tarantula spider Thrixopelma pruriens, modifies gating in voltage-gated Na + and Ca 2+ channels. ProTxII is distinct from other known Na + channel gating modifier toxins in that it affects activation, but not inactivation. It shifts activation gating positively and decreases current magnitude such that the dose-dependence of toxin action measured at a single potential reflects both effects. To test the extent to which these effects were independent, we tracked several different measures of current amplitude, voltage-dependent activation, and current kinetics in Na V1.5 in a range of toxin concentrations. Changes in voltage dependence and a decrease in G max appeared at relatively low concentrations (40–100 nM) while a positive shift in the voltage range of activation was apparent at higher toxin concentrations (≥500 nM). Because ProTxII carries a net +4 charge we tested whether electrostatic interactions contributed to toxin action. We examined the effects of ProTxII in the presence of high extracellular Ba 2+, known to screen and/or bind to surface charge. Some, but not all aspects of ProTxII modification were sensitive to the presence of Ba 2+ indicating the contribution of an electrostatic, surface charge-like mechanism and supporting the idea of a multi-faceted toxin-channel interaction.